A normally-off semiconductor switching element made of gallium nitride (GaN), which is one of a power device, is capable of performing a high-speed switching operation. However, because a gallium nitride element has a threshold voltage of, for example, about 1 V, which is lower than a threshold voltage of a normal FET mainly made of silicon, a malfunction is likely to be caused by influence of noise in association with a switching operation.
Thus, in a gate driving circuit of a gallium nitride element, an off-holding voltage is set to be negative with respect to a source of the gallium nitride element, so as to restrict a malfunction. In such a case, while a gate driving circuit of the normal FET uses a positive single power source, the gate driving circuit of the gallium nitride element requires an additional negative power source for the off-holding at the negative voltage. Thus, an increase in cost is inevitable, and a size of the gate driving circuit also increases.
On the other hand, JP 2009-21823 A (corresponding to US 2009/0033377 A1) and JP 2000-59195 A disclose to generate a negative voltage from a single power source by adding a charge pump function to a gate driving circuit, instead of providing a negative power source. Accordingly, in the gate driving circuit generating the negative power source, the negative voltage is the same voltage as the single power source. However, when the negative voltage for the off-holding is applied to the gallium nitride element as an offset at a reverse conduction, a loss at the reverse conduction increases. Furthermore, because a semiconductor switching element is added for generating the negative voltage, a control may be complicated.